Accurate measurements of parameters (such as for example force) are often required in a wide variety of applications. Micro-electromechanical sensors (MEMS) devices such as accelerometers have been extensively used in, e.g., dynamic distance and speed measurements, inclination, machine vibration, buildings and structural monitoring, component placement in manufacturing, process control systems and safety installations. Angular rotation rate MEMS (also referred to as the gyroscope or the rate sensors) are useful in, inter alia, navigation, automotive (e.g., electronic stability control), entertainment (e.g., user motion detection for game consoles), photography (e.g., image stabilization), animal behavior studies and many other applications. Pressure sensors are similarly widely used in applications such as weather, industrial monitoring and control, aircraft and automotive, oil and gas exploration, flow sensing, acoustics, etc. Many other parameter measurement applications exist (such as for example, magnetic force measurements used in navigation and mineral exploration, or electrostatic force measurements used in microscopy, etc.).
In the context of a force measurement, the typical prior art force sensor measures displacement (also often referred to as “deflection”) of a spring-suspended proof mass in order to estimate a force acting on the proof mass. The methods of measuring such deflection vary in accuracy, variability, and cost of implementation. Various measurement approaches may be used, such as for example capacitive, piezo-resistive, electron tunneling sensing, and optical interferometery, in order to determine the proof mass deflection. In all of these approaches, the deflection (and thus the force) is inferred as a function of a measured voltage (or electric current), and therefore is inevitably subject to measurement errors due to, inter alia, thermal and electromagnetic noise. As a result, most existing force sensor solutions require very accurate signal conditioning circuitry (such as precision amplifiers, filters, voltage references, etc.), as well as periodic recalibrations to account for sensor aging (including e.g., changes in the physical properties or characteristics of the “spring” and/or proof mass with time), and electrical component drift.
As an alternative, perturbation analysis can be accomplished in the time domain, as opposed to measure the deflection of a MEMS device. Time domain switched inertial sensors in the prior art can typically work by assuming that the displacement of a harmonically oscillating proof mass on a spring is going to be the sum of the sinusoidal harmonic oscillation and the displacement due to an external force. By curve fitting time intervals between known displacements to this anticipated behavior, the extraction of the value of the external force can be determined. If the external force is constant for the period of the harmonic oscillation, a simple cosine plus offset curve fit can be used. However, if the force is changing significantly during the period of the oscillation, a more computationally intensive polynomial fit should be used. If the amount of time the force must be constant could be minimized, the simple cosine curve fit could be used with greater accuracy.
One way to allow the use of the simple cosine plus offset curve fit with greater accuracy can be by greatly reducing the time interval during which the force needs to remain near constant. This can be done by imposing two oscillations on the device, and then measuring the beginning of an interval from the crossing past a reference past due to one harmonic oscillation while measuring the end of the interval when the device crosses the reference point due to an identical harmonic oscillator (same amplitude and period) that has a known phase shift from the first harmonic, for example, oscillating 180 degrees out of phase from the first.
If the two oscillations are identical in period and amplitude, but have a known phase shift, triggering information from one oscillation should be identical to the other after period of time associated with the phase shift. By using the virtual interval determined by adding the phase difference to the triggering of the two out of phase oscillations, the time period for which the offsetting influence is needed to be relatively constant become much smaller. This improves the accuracy of the measurement. This can work for two 180 degrees out of phase sinusoidal oscillations, or can work by having the perturbation added and subtracted to identical sinusoidal oscillations.
In view of the above, it is an object of the present invention to provide a MEMS device that performs the measuring function using perturbation analysis of harmonic oscillations in the time domain. Another object of the present invention is to provide a time domain switched inertial sensor (TDSIS) that measures time intervals between harmonic oscillations of the proof mass instead of displacement, in order to determine inertial force. Another object of the present invention is to provide a TDSIS that minimizes that time required for the inertial force to be nearly constant. Yet another object of the present invention is to provide a TDSIS with improved accuracy. Still another object of the present invention is to provide a TDSIS with simplified structure, which can be manufactured and used in a cost-efficient manner.